A spring unit for an upholstered article comprises an array of interconnected helical coil springs formed from metal wire.
The production of such a spring unit conventionally comprises three principal steps that are described below with reference to FIG. 1.
First the wire is coiled to form the springs. In order to do this, wire 1 from a reel 2 is fed in the direction of arrow A to a coiling machine 3 to form a coiled wire 4 consisting of a continuous series of alternating left and right-handed helical coils 5,6 interposed with substantially straight sections of wire 7. The coiled wire 4 is folded at appropriate intervals as it emerges from the coiling machine so that the straight sections of wire 7 are parallel to one another and adjacent left and right-handed coils 5,6 are arranged so that their central longitudinal axes are approximately disposed in parallel.
The folded coils 4 are fed to a linking table 8 where the adjacent right and left-handed coils are interlinked. The strings of coils 9 are periodically cut into predetermined lengths and each string 9 fed on to a storage reel 10 ready for use in the final step of the process. To form the complete spring unit, the strings of coiled wire 9 are fed from a plurality of such storage reels 10 via channels 11 defined between dividers 12 to a spring unit assembly machine 13 where the strings 9 are interconnected to form the finished spring unit. In an alternative embodiment, sets of folded coils 9 exiting a plurality of folding tables 8 may be fed directly to the spring unit assembly machine 13 via channels 11.
The assembly machine 13 advances the strings 9 in parallel such that the coils 14 are aligned. The strings 9 are indexed by one coil width at a time to a set of transversely extending jaws 15 between which they are clamped. Successive coils 14 in the adjacent strings 9 are clamped with their longitudinal axes substantially upright. The jaws 15 effectively form a continuous helical channel into which a helical binding wire 16 is advanced. The binding wire is formed by passing uncoiled wire 17 from a reel 18 to a coiling passage 19 located to the side of the jaws 15 of the assembly machine 13. It is rotated and axially advanced in the transverse direction of arrow B through the jaws 15 such that is passes around the wire of the adjacent strings 9 and so as to form a row 20 of bound coils 14. The jaws 15 are then opened and the joined strings of coils 9 indexed forward in the direction of arrow A so as to locate the next coil of each string 9 within the jaws 15 whereupon the above cycle is repeated to bind the next row of coils together. The binding cycle is repeated a sufficient number of times to bind a suitable number of rows of coils together to produce a spring unit of the desired size.
One example of a method for manufacturing the strings of coils prior to the assembly machine is described in U.S. Pat. No. 5,105,642. This method is unduly complex particularly as it includes an additional folding station between the coiler and a coil interlock station. There is no detailed description of interlocking method. A problem with a coiler of this kind is that adjustment of the coil pitch is not possible without significant changes to the relative positions of the machine components.
An example of a conventional process for interlinking adjacent left and right handed coils comprises passing the coiled wire to a linking table whereupon a straight section of the wire interposed between the coiled sections is presented to a pivotable butterfly clamp which is located centrally with respect to the table. The straight section of the wire is then held in place by the butterfly clamp with the left and right handed coiled sections to either side. One of the coiled sections is then engaged by a ‘pecker arm’ which moves transverse to the longitudinal axis of the table to engage the coil and hold it in place relative to the linking table. A folding arm mounted above the table surface is then operated to pivot about a substantially upright support member and engage the free coiled section of wire on the opposite side of the butterfly clamp. Pivoting of the folding arm draws the free coiled section in an arc around the butterfly clamp towards the other coiled section which is held by the ‘pecker arm’ to interlink the two coiled sections of wire.
The process is unduly complex and requires extremely accurate control of a number of different simultaneous actions. Due to the complicated manner in which adjacent coils are interlinked, the operational efficiency of the process is severely restricted. For example, a process of this kind could typically interlink only 30 to 35 coils per minute. The apparatus required to carry out the process incorporates a number of different cammed surfaces to accurately control the movement of the various components. A problem with linking tables of this kind is that adjustment of the various components to accommodate coils of different sizes is not possible without significant changes to the relative positions of the machine components and the complicated nature of the apparatus results in reliability problems.
An example of an assembly machine is described in EP0248661. The disadvantage of this machine is that each of the pairs of jaws are opened and closed by a respective double acting pneumatic piston. Such a piston has at least one sensor so that the opening and closing of the jaws can be monitored. In operation it has been found that the machine operation is often interrupted through the malfunction of at least one sensor. The use of so many sensors increases the scope for interruption of the machine operation. Moreover, since the piston stroke time (and therefore the time required to open and close a pair of jaws) varies between pistons a sufficient time window has to be built into the timing cycle of the assembly operation in order to be sure that all of the jaws have opened or closed.